LegacyInjector.writeJar(): handle ImageJ 1.x in a .jar file gracefully

So far, this topic branch was only tested in Eclipse, with ImageJ 1.x in
the workspace, i.e. available as an unpacked directory. Things change a
bit when running it outside, on .jar files.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Try to call FijiInitializer before LegacyInitializer

The default class to use as initializer in case the property
ij1.patcher.initializer is not set was the LegacyInitializer from the
ij-legacy artifact. However, when writing a fully patched ij.jar, we
will want to try FijiInitializer first (which will eventually cease to
exist in favor of moving everything into the LegacyInitializer context).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>